1. Field of the Invention
The present invention relates to longitudinally pumped laser oscillators, and particularly to lasers with astigmatism-compensated resonant cavities.
2. Description of Related Art
A variety of laser gain media, such as Ti:Sapphire and Co:MgF.sub.2, have been used in longitudinally pumped laser oscillators. In such oscillators, a pump beam and the cavity mode propagate collinearly over the length of the gain media for efficient transfer of pump power to laser beam output power. One embodiment of such oscillators is the form of a folded cavity with a rod-shaped laser media having Brewster-cut faces. The folded cavity induces astigmatism in the cavity mode and the Brewster-cut faces of the laser media rod induce astigmatism in both the cavity mode and the pumping beam within the gain media.
It has been recognized that the astigmatism in the cavity mode can be compensated by proper selection of the Brewster element thickness, and of the focal length and incident angle of the off-axis spherical mirrors in folded cavities (See Kogelnik et al., "Astigmatically Compensated Cavities for CW Die Lasers", IEEE Journal of Quantum Electronics, Vol. QE/8, No. 3, March 1972). The Kogelnik work is directed to compensating the astigmatism induced by off-axis focusing mirrors through the insertion of a Brewster-angled plate into the focused beam. However, this work does not address the additional requirements of pump and cavity mode matching within a Brewster-plate serving as a gain medium.
In longitudinally pumped lasers, contrary to what has been reported by Moulton in "An Investigation of the Co:MgF.sub.2 Laser System", IEEE Journal of Quantum lo Electronics, Vol. QE/21, No. 10, October 1985, the pump and laser beams are not identically diffracted within the laser medium due to their differing wavelengths. Therefore, the problem of matching the pumped region within the laser media to the cavity mode volume within the gain medium has not been adequately addressed in the prior art.
This problem has been recognized by Schulz, "Single-Frequency Ti:Al.sub.2 O.sub.3 Ring Laser", presented at CLEO Conference, April 1987. According to Schulz, the output power of a ring laser described in the paper can be increased around ten percent (10%) by inserting a slightly rotated, thin convex focusing lens in the path of the pumping beam outside the resonant cavity of the laser. Rotating the lens induces astigmatism in the pumping beam that improves the match of the pump mode with the cavity mode within the amplifying medium. However, use of the rotated, focusing lens of Schulz also induces a coma effect (tear-shaped distortion) in the pump mode within the medium, resulting in a poor match with the astigmatic cavity mode. In addition, such lenses suffer chromatic dispersion, which could have a slight effect on their practicality over a wide pumping range. As is evidenced by the slight 10 percent gain in output power achieved by Schulz, the rotated lens does not provide a good match of pumped area to cavity mode volume within the gain medium.
It is desirable to obtain the best average match of the pump volume with the cavity mode volume within the gain medium and otherwise to optimize the output power of longitudinally pumped lasers.